Growth in tree canopy across Chicago neighbourhoods was associated with lower mortality between 2011 and 2021, Northwestern University reported.
Researchers compared annual satellite measurements of tree canopy in every city census tract with more than 220,000 death records. Their model accounted for summer temperatures, air pollution, income, population size and other neighbourhood characteristics.
Overall canopy coverage alone had no statistically significant association with mortality. Change over time mattered more: annual canopy gains were linked to fewer deaths, while tree loss was linked to more.
The strongest associations involved cardiovascular and respiratory disease, several musculoskeletal conditions and mental health-related illnesses. In the hottest neighbourhoods, canopy losses coincided with higher mortality.
“Previous studies have mostly looked at how health outcomes are associated with static tree canopy coverage. But cities and their tree canopies constantly change, so we looked at how the canopy and death rates change over the years. We found that as tree canopy decreases, mortality increases.” — Daniel Horton, senior author of the study.
The team estimated that expanding canopy by about 0.03% a year — roughly 14,000 new trees — was associated with approximately 105 fewer deaths annually.
The authors published the study in GeoHealth. They say cities should not only plant saplings but also protect mature trees that already provide broad shade and cool streets.
